We are investigating phenylalanine hydroxylase, tyrosine hydroxylase, tryptophan hydroxylase, and nitric oxide synthase. Each of these enzymes requires the cofactor, tetrahydrobiopterin. We are also studying the enzymes involved in the biosynthesis of tetrahydrobiopterin. Our attack has been multidisciplinary and includes recombinant DNA techniques linked with enzymology. With the aid of high-level expression systems and directed mutagenesis, we have discovered interactive domains that are important for the catalysis and regulation of these enzymes in normal and diseased individuals.